This may happen from dryness or 'cracked nipple'.
You can try eucerin cream topically & see if you feel better.
Another possibility is fungal infection or candidosis.
This responds to topical anti fungal cream like clotrimazole ointment.
Nipple adenoma, a benign condition
Toker cell hyperplasia, another benign condition--- are more common.
Paget's disease or certain type of skin cancers like bowen's disease should be considered if the the condition is persistent & worsening.